The Red Devils took an early two-goal lead thanks to strikes from Chicharito and Wayne Rooney but were pegged back by a second-half comeback from the visitors, with Eden Hazard and Ramires forcing a quarter-final replay.

However, speaking to reporters after the final whistle, Sir Alex was not disappointed by the final result and attributed part of the team's second-half performance to the controversial loss to Real Madrid on Tuesday.

"We're lucky to be in the cup," the Scot admitted to reporters. "We just ran out of legs and Chelsea were far the better team in the second half.

"But I can't be critical of my side because the running they had to do on Tuesday night, against a really top side in Real Madrid, had its bearing on today's result, no doubt about that.

"The two full-backs, Rafael and Patrice [Evra], are knackered and the two central midfielders tired badly. We kept giving the ball away and it made it a long day for us, going from one penalty box and back to our own."

If United are to overcome Chelsea in the replay at Stamford Bridge they will face Manchester City at Wembley - the victors in a semi-final between the local rivals in 2011.

"The replay is not a problem for us," Sir Alex told MUTV. "We've got plenty of time for it as we have no European games.

"I don't know when it's going to be played mind you - probably May because we've got the international break coming up! But the only thing we can say is we're still in the cup."